That portion of an image being projected onto a presenter in front of a projection screen is distorted and very distracting to both an audience and a presenter. U.S. Pat. No. 6,361,173 describes a method for generating a matte (alpha) signal to selectively inhibit the projected image in the screen area occupied by a presenter. A presenter may then walk in front of the screen and look at his audience without being blinded by the projector.
The screen and presenter are illuminated with infrared light, and observed with an infrared camera. The pixel levels of a stored infrared screen-image when subtracted from the camera image of the screen with presenter present are zero in the unobscured screen area. All other pixels should be in the silhouette area of the presenter. Several other methods are described for locating the position of the presenter.
The increased infrared illumination from opening a door or adjusting room illumination can result in a failure to generate a useable matte; also a portion of the presenter, or his wardrobe, matching the screen""s infrared reflection can destroy a portion of the matte.
To develop a reliable binary selective inhibit signal accurately identifying those pixels comprising the presenter""s silhouette; where said signal is immune to a subject""s infrared reflection levels and immune to randomly occurring non-uniform infrared flooding of the projection screen and presenter.
A pattern is projected onto a screen by an infrared projector. An infrared camera, displaced several inches from said projector, observes the projected pattern whose signals are then stored to create a reference frame. Infrared signals from a current frame, including a presenter, and signals obtained from said reference frame, are compared to determine their difference. In unobscured screen areas, the pattern images match. The infrared pattern on the presenter is displaced because of the displaced camera, and no longer matches the reference pattern, thereby identifying the presenter""s silhouette area.